Wind turbines convert wind energy to electrical energy by using the wind to drive the rotor of generator, either directly or by means of a gear box. The frequency of the AC power that is developed at the stator terminals of the generator is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as function of speed and, depending on the particular type of generator, on the flux level. For optimum power, the speed of rotation of the output shaft of the wind turbine will vary according to the speed of the wind driving the wind turbine blades. To limit the power at high wind speed, the speed of rotation of the output shaft is controlled by altering the pitch of the turbine blades.
Power converters are, for example, used for matching the variable frequency AC power provided by the generator to the nominally constant frequency AC power of the grid. In a first stage of such a power converter a rectifier is used to convert the AC power delivered from the generator to a DC power. This DC power is fed to a so called DC link. In a second stage, called inverter, the inputs of which are connected to the DC link, the DC power is converted to an AC power matching the grid frequency.
In general, the inverters are circuits used for converting DC power to AC power. They are equipped with switches for connecting the inverter phase terminal outputs to the positive or negative busbar of the DC link. The switching pattern for opening and closing the switches is provided on the basis of a pulse width modulation scheme which defines the timing for connecting the inverter outputs to the high or the low DC voltage level through the respective switches. During the times in which a switch is open a current flows from the DC link to the grid or vice versa. Other methods for determining the switching regime for the switches including direct control or predictive control are also applicable.
The inverter may be controlled according to a power factor demand signal representing a power factor requested by the grid operator, where the power factor is given by the ratio of the real power to the apparent power (the apparent power is the square root of the sum of the squares of the real power and the reactive power). Instead of controlling the inverter according to a power factor demand the inverter can also be controlled on the basis of a real power demand and a reactive power demand. Furthermore, instead of controlling the inverter directly according to a power factor demand, or a real power demand and reactive power demand, it can also be controlled according to current demand signals since the voltage amplitude is usually a fixed parameter in the grid so that the power fed to the grid by the inverter can be defined by current amplitudes and phase angles between the current and the voltage. Hence, a power factor demand signal or demand signals for active and reactive power can be converted to current demand signals which are then used for controlling the inverter, i.e. for determining the pulses defining the switching times of the switches. Such mode of control is known as current control.
In particular, in the current control mode a vector control algorithm may advantageously be used for controlling conversion of DC power into AC power. Balanced alternating feedback signals, acquired in a stationary reference frame, can be represented as components Iα and Iβ of vectors I rotating with a rotational frequency ωr with respect to the stationary reference frame α,β (compare FIG. 10). In a vector control algorithm, these rotating vectors I are represented in a rotating reference frame rotating with the flux vector of the rotor or a vector rotating with the grid voltage (synchronised to the grid voltage) and the control quantities are calculated in this rotating reference frame based on current demand signals. In such a rotating reference frame a balanced three phase AC current I can be defined by two vectors vector components Id, Iq representing a current Id flowing in the direction of the flux vector, i.e. the so called direct axis of the rotating reference frame, and a current Iq flowing perpendicular to the direction of the flux vector, i.e. in the direction of the so called quadrature axis of the rotating reference frame. While the current Id flowing in the direction of the flux vector or the vector rotating with the grid voltage is called direct axis current the current Iq flowing perpendicular to the flux vector or the vector rotating with the grid voltage is called quadrature axis current. The direct axis current and the quadrature axis current are DC quantities in the steady state, and the state error is controlled to zero, typically by means of a PI controller.
Typical power converters including a rectifier and an inverter for converting a variable frequency AC power of a wind turbine generator to a DC power and the DC power to an fixed frequency AC power of a grid on the basis of vector control algorithms are, for example, described in U.S. Pat. No. 5,083,039 and US 2009/0147549 A1.
The mentioned controllers typically have a frequency dependent open loop control loop gain. In case the open loop control loop gain is greater than unity and has a phase less than or equal to minus 180° the control system is defined to be unstable.